Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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861 FXUS66 KLOX 230721 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1221 AM PDT Thu May 23 2024 .SYNOPSIS...22/919 PM. Strong onshore flow and a deep marine layer with a passing upper- level trough will continue to keep low clouds and fog spreading into coastal slopes of the mountains the next couple of days. Night through morning drizzle will be possible the next couple of night and mornings. Clouds will struggle to clear from the beaches each day through Saturday. A warming trend will establish over the weekend as high pressure aloft builds into the region. Better clearing and near normal temperatures are expected by the middle of next week. && .SHORT TERM (THU-SUN)...22/953 PM. Low clouds are advancing rapidly inland this evening as strong onshore flow deepens the marine intrusion. The onshore push, brought to us by broad upper-level trough aloft, will continue to tighten through Friday. A shortwave trough will move south over the region into Thursday, then a stronger but moisture-starved will drop down out of the Gulf of Alaska through Friday. A cooling trend will continue into Friday away from the coast. Coastal locales will likely remain near persistence over the next couple of days. Low clouds and fog will be staple of the forecast through at least Friday or Saturday. With the strong onshore pressure gradients between 8 and 10 mb over the next several days, clouds will very likely struggle to clear away from the land mass each afternoon. There is a slight chance that enough cold air advection could sneak in to bring better clearing north of Point Conception, but local studies suggest the strong pressure gradients will likely win out and keep a cloudier forecast in play. The instability with the shortwave trough could lift the marine layer deck and squeeze out some drizzle tonight and into Thursday morning. EPS ensemble members and high-resolution multi-model ensemble members do have some solutions with drizzle or very light rain developing at some sites through Thursday morning. As the stronger trough approaches, there is some concern that the drizzle or light rain on Thursday night and Friday morning could turn into light rain across the eastern Los Angeles County. PoPs may be need to increased to account for this. The previous forecast did introduce mountain showers, but there is a non-zero chance for light rain across the San Gabriel Valley, as well. With the strong onshore gradients, gusty onshore winds will develop across the interior portions of the area Thursday afternoon and evening, and again Friday afternoon and evening. The strongest winds appears to be on Friday afternoon and evening when a wide area of advisory level winds could develop across the region. The highest confidence in gusty winds for Thursday will be for the western Antelope Valley foothills, but ensemble members hint that advisory level winds could not only develop across the interior, but also across the Central Coast, southern Santa Barbara County, and into the Ventura County coast. Future shifts will need to take a closer look at this potential. An update was issued earlier to tweak temperatures and cloud coverage for Thursday. No additional updates are planned at this time. ***From Previous Discussion*** Friday, a sharper shortwave trough will quickly pass over the region, bringing colder air aloft. This will further increase chances of drizzle and fog overnight into the morning. For most locations except for the coasts, Friday will be the coldest day of the forecast period, with most maximum temperatures in the 60s, and deserts only reaching the 70s. These temperatures are 6 to 12 degrees below normal. Clearing is generally expected to be even slower, if at all. However, there is a 20% chance that the instability will cause fragmentation of the marine layer, resulting in pockets of sunshine and slightly warmer temperatures than forecast, even at the beaches. In addition, instability from the trough will lead to a 10-15% chance of afternoon shower/thunderstorms for inland mountain ranges. Finally, advisory level gusts are possible during the afternoon and evening, for the western Antelope Valley and foothills and the western portion of the Santa Barbara South Coast. Saturday the trough will start breaking down and upper level heights will rise. Highs will a few degrees warmer than Friday, kicking off a warming trend, and clouds are expected to be less extensive, as higher upper level pressure compresses the marine layer. .LONG TERM (MON-THU)...22/213 PM. The GFS and ECMWF and their respective ensemble means continue to show little variation in the pattern for Sunday through Tuesday. Sunday the persistent broad trough will finally break down, giving way to a weak ridge. Onshore flow will weaken at this time, and thus the inland reach of marine layer clouds is expected to subside each day as upper level heights rise. Daytime chances for clearing and sunshine will improve for the latter portion of Memorial Day weekend into Tuesday. Maximum temperatures will trend upwards, and will likely peak Tuesday or Wednesday. Daytime highs for Memorial day will range from the mid 60s to mid 70s for coasts, and up to the low 80s for valleys. This will put temperatures just a touch below normal. Additionally warming on Tuesday will increase conditions to near normal for most locations. Wednesday marks the start of mild disagreement between various models. NBM clusters show the potential for the weak ridging pattern to start to break down. The GFS leans towards falling hieghts, while the ECMWF continues the weak ridge. The overall impacts of these differences are expected to be minimal for Wednesday, mostly impacting the extent of marine layer clouds and slightly impacting high temperatures. && .AVIATION...23/0719Z. At 0505Z at KLAX, the marine layer was 2800 ft deep. The top of the inversion was at 5400 feet with a temperature of 16 C. High confidence in desert TAFs. Moderate confidence in TAFs. There is a 20 percent chc of brief SCT conds at any site with no clearing fcst. There is a 30 percent chc of no IFR cigs at sites with them in the fcst. KLAX...Moderate confidence in TAF. There is a 20 percent chc of a few hours of SCT conds in the afternoon. Any east wind component will be 6kt or less. KBUR...Moderate confidence in TAF. VFR transition could occur any time 18Z-21Z. && .MARINE...22/823 PM. For the Outer Waters, moderate to high confidence in current forecast. Tonight through Saturday, there is a 70-90% chance of Small Craft Advisory (SCA) level winds and seas. For Sunday and Monday, there is a 30-50% chance of SCA level winds. For the Inner Waters north of Point Sal, moderate confidence in current forecast. Tonight through Thursday morning, high confidence in winds and seas remaining below SCA levels. For Thursday afternoon through Saturday night, there is a 40-50% chance of SCA level winds, mainly in the afternoon and evening hours. For the Inner Waters south of Point Conception, moderate to high confidence in current. For most of the area, high confidence in winds and seas remaining below SCA levels through Monday. However across the western half of the Santa Barbara Channel, there is a 30-40% chance of SCA level winds through Thursday then a 60-70% chance on Friday and Saturday. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Wind Advisory in effect from 2 PM this afternoon to 9 PM PDT this evening for zone 381. (See LAXNPWLOX). PZ...NONE. && $$ PUBLIC...Hall/Schoenfeld AVIATION...Rorke MARINE...RAT/Lund SYNOPSIS...Hall weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox